In the field of machinery, the so-called fiber-reinforced plastics including a matrix resin and reinforcing fibers, e.g., carbon fibers, are attracting attention in recent years. Since these fiber-reinforced plastics are excellent in terms of tensile modulus, tensile strength, impact resistance, etc. due to the reinforcing fibers dispersed in the plastics, and use thereof in applications such as structural members for motor vehicles or the like is being investigated. These fiber-reinforced plastics can be formed into desired shapes using compression molding, or the like.
However, fiber-reinforced plastics have had a peculiar problem in that since the fibers have mostly been aligned not in the thickness direction, these fiber-reinforced plastics are not always excellent in terms of thickness-direction mechanical strength, specifically, compressive strength and ability to absorb impact energy. Although it is, of course, possible to improve these properties by increasing the material thickness, it has been pointed out that the increased thickness reduces design choices or results in a problem in that the resultant increase in weight impairs the intrinsic attractiveness of fiber-reinforced plastics.
In view of such problems, patent document 1 proposes a method in which fibers in a fiber-reinforced plastic are aligned in the thickness direction by a needle punching method. This needle punching method has an advantage in that thickness-direction compressive strength, etc. can be improved by stabbing an unformed fiber-reinforced plastic material with needles to arrange the reinforcing fibers within the needle-stabbed portions so that the longitudinal directions of the reinforcing fibers are aligned in the thickness direction. However, the following problem has been pointed out. In the needle punching method, there are often cases where when needles are stabbed into an unformed fiber-reinforced plastic material, the reinforcing fibers are damaged or broken. Because of this, the thickness-direction mechanical strength improves at the sacrifice of in-plane-direction strength.
In order to suppress the decrease in in-plane-direction strength which is the problem of patent document 1, patent document 2 proposes a method in which a precursor for reinforcing fibers is crimped, entangled by a needle punching method or the like, and then burned to produce a nonwoven fabric of reinforcing fibers and a fiber-reinforced plastic is obtained therefrom to thereby maintain the in-plane-direction strength. However, even in this method, the problem wherein the in-plane-direction strength undesirably decreases remains unimproved since some of the reinforcing fibers are oriented in the thickness direction. In addition, in such a method, not only the reinforcing fibers themselves become bulky undesirably, but also some degree of material thickness is necessary for maintaining strength. It has hence been pointed out that the proposed method is prone to result in a decrease in formability and induce an obstacle to thickness reduction.
Furthermore, patent document 3 discloses a composite material in which the impact strength and other properties have been improved by using continuous fibers as the only reinforcing fibers and wavily disposing continuous fibers so that all the continuous fibers match in wavelength with each other. However, in the composite material of patent document 3 since continuous fibers are used as the only reinforcing fibers and the continuous fibers have been made to match in the wavelength direction of the wavy shape, there is a problem in that this composite material undesirably has considerably reduced, rather than increased, strength in directions perpendicular to the wavelength direction. In addition, the following problem has been pointed out: since continuous fibers are used as the only reinforcing fibers, the composite material, when the reinforcing fibers are wavily arranged, unavoidably come to include portions in which reinforcing fibers are absent and which are constituted by the resin only. Consequently, such portions are causative of unevenness in strength, or the like.